When the Background property of a window is set to a transparent color, using Transparent for example, the window remains opaque. This means that the desktop and any running applications "beneath" the window cannot be seen. To enable this type of transparency, AllowsTransparency must be set to true.

AllowsTransparency exists to facilitate the creation of non-rectangular windows, and, consequently, when AllowsTransparency is set to true, a window's WindowStyle property must be set to None.